Typically prostate cancer is diagnosed after closely examining biopsy cells through a microscope. There are several types of cells in the prostate, and each contributes in its own way to the prostate's development, architecture, and function.
But cancer cells look different than normal prostate cells. Pathologists look for these differences first to detect the presence of cancer and then to determine the cancer grade.

Gleason Grading
The Gleason grading system accounts for the five distinct patterns that prostate tumor cells tend to go through as they change from normal cells to tumor cells.
The cells are scored on a scale from 1 to 5:

"Low-grade" tumor cells
(those closest to 1) tend to look very similar to normal cells.

"High-grade" tumor cells
(closest to 5) have mutated so much that they often barely resemble
the normal cells.

The Gleason Score

The pathologist looking at the biopsy sample assigns one Gleason grade to the most similar pattern in your biopsy and a second Gleason grade to the second most similar pattern. The two grades added together determine your Gleason score (between 2 and 10). Cancers with lower Gleason scores (2 - 4) tend to be less aggressive, while cancers with higher Gleason scores (7 – 10) tend to be more aggressive.
It's also important to know if any Gleason 5 is present, and most pathologists will report this. Having any Gleason 5 in your biopsy or prostate puts you at a higher risk of recurrence.